1. Field of the Invention
The present invention relates to a knock control apparatus that controls a knock occurring in an internal combustion engine.
2. Background Art
There has been a method of detecting a knock phenomenon occurring in an internal combustion engine using a vibration sensor (hereinafter, referred to as the knock sensor) directly installed to the block of the internal combustion engine. When a knock occurs while the internal combustion engine is running, vibrations in a specific frequency band corresponding to a bore diameter of the internal combustion engine and a vibrational mode of a knock are known to occur. A knock is therefore detected by measuring vibration intensity at this specific frequency.
There is also a knock control method of suppressing a knock by correcting a spark timing to the retard side when a knock is detected and minimizing a torque reduction by returning back a spark timing to on the advance side when no knock is detected. As a characteristic of the internal combustion engine, it is known that an output torque of the internal combustion engine is enhanced whereas a knock occurs readily when a spark timing is advanced and conversely an output torque of the internal combustion engine is reduced whereas a knock hardly occurs when a spark timing is retarded. The internal combustion engine is therefore controlled to run at a knock limit spark timing at which the largest torque is generated while suppressing the occurrence of a knock by correcting a spark timing to be on the retard side when a knock is detected and by returning a spark timing to be back on the advance side when no knock is detected. It should be noted, however, that there is a case where no knock occurs when a spark timing is advanced to the extent at which a torque reaches the maximum while the internal combustion engine is run by a low load. The knock control described above is not necessary in such a running region.
In a case where such knock control is performed, a spark timing correction method is necessary to suppress a knock in a reliable manner when a knock is detected and also to minimize a torque reduction when no knock is detected. In particular, because a compression ratio tends to be set high (this causes a knock to readily occur) in these days to enhance fuel efficiency of the internal combustion engine, such knock control is becoming more crucial. As such a method, there have been proposed a method of minimizing a torque reduction by changing a return rate to the advance side when no knock is detected and a method of reducing a spark timing correction amount by which the spark timing is further retarded when a knock is detected in a case where the spark timing has been corrected to the retard side.
More specifically, Patent Document 1 discloses a spark timing control method for an internal combustion engine by which a spark timing is corrected to the retard side by a predetermined amount when a knock is detected and when no knock is detected, a return rate to the advance side is increased when a spark timing correction amount is greater than a predetermined value and a return rate to the advance side is decreased when the spark timing correction amount is smaller than the predetermined value.
Also, Patent Document 2 discloses a spark timing control apparatus for an internal combustion engine that calculates a retard correction amount according to knock intensity when a knock is detected and magnitude of a retard correction amount at that time and lowers a return rate to the advance side with time when no knock is detected.
Further, Patent Document 3 discloses a knock control apparatus for internal combustion engine that changes a retard correction amount when a knock is detected and a return rate when no knock is detected according to a spark timing and a learning value.
Patent Document 1: JP-B-3-20593
Patent Document 2: JP-A-63-80074
Patent Document 3: JP-A-2005-127154
The method in the related art disclosed in Patent Document 1 achieves an advantage that a torque down amount is suppressed in comparison with a case where a return rate is constant when an over-retarded spark timing is returned back to the advance side. However, because a spark timing has been corrected to be on the retard side by a predetermined amount when a knock is detected, there are a problem that knocks occur successively due to an insufficient retard correction amount depending on the magnitude of a knock and a problem that excessive torque down or torque fluctuation occurs due to an excess retard correction amount. Also, there is another problem that it is necessary to match the magnitude of a spark timing correction amount by which a return rate is changed to the advance side.
The apparatus in the related art disclosed in Patent Document 2 achieves an advantage that an adequate retard correction amount is obtained because a retard correction amount corresponding to knock intensity and the magnitude of a retard correction amount at this time can be obtained. However, for a large knock occurring unexpectedly in the vicinity of the knock limit spark timing, although it is thought that such an advantage can be achieved by some measure, a retard correction amount is still excessive, and there is a problem that a torque down occurs excessively. Also, there is another problem that when a return rate to the advance side is lowered with time, the spark timing is returned back to the advance side with a delay when a retard correction amount is excessive, thereby causing a torque down to occur excessively.
Further, as has been described, there is a case in these days where a compression ratio is set high in order to enhance fuel efficiency of the internal combustion engine. However, a knock occurs more readily in an internal combustion engine set at a high compression ratio (for example, a compression ratio of about 14:1) than in an internal combustion engine set at a normal compression ratio (for example, a compression ratio of about 10:1). Such being the case, the basic spark timing tends to be set on the retard side where a gradient of the torque characteristic is steep. Hence, in an internal combustion engine set at a high compression ratio, when a retard correction amount is calculated on the basis of the knock intensity upon detecting a knock, there is a tendency that a torque down amount increases more than in a case where the retard correction amount is calculated in the same manner for an internal combustion engine set at a normal compression ratio. As has been described, in an internal combustion engine set at a high compression ratio there is a problem that a toque down amount readily increases, that is, a spark timing is readily retarded excessively in comparison with an internal combustion engine set at a normal compression ratio even when the retard correction amount is the same.
Further, the apparatus in the related art disclosed in Patent Document 3 also has the same problems as those of the method and the apparatus in the related art disclosed in Patent Documents 1 and 2.